An American agent has tracked down the stronghold of an evil criminal mastermind, determined to take over the world (what, another one ?).

At the dawn of the 20th century, following their father's arrest on suspicion of betraying state secrets, the three Waterbury children—Bobbie, Phyllis and Peter—move with their mother to Yorkshire, where they find themselves involved in unexpected dramas along the railway by their new home.

During an annual board of trustees meeting, one of the trustees dies. Miss Marple thinks he’s been poisoned after finding a chemical on him. She sets off to investigate at the ship where he had just come from. The fourth and final film from the Miss Marple series starring Margaret Rutherford as the quirky amateur detective.